The present invention relates to insulation condition monitoring, and more specifically to a technique for monitoring of the state of insulation, such as for stator winding insulation condition monitoring.
Many different types of electrical machines employ windings and other conductors that are insulated by various insulation systems. For example, motors and generators commonly include wound rotors and stators, or may include preformed windings or coils. The conductors typically serve either simply to conduct electrical current, or to produce magnetic fields by virtue of the flow of such current. Insulation systems separate conductors and windings from one another, and from adjacent components in the assembled system. Such insulation systems may include various varnish systems, tapes, coatings, sleeves, and so forth, or combinations of these. The integrity of the insulation systems is important to the reliability and life expectancy of the electrical equipments in which they are installed.
Insulating systems may break down for many reasons, jeopardizing the continued operation of electrical equipment. The winding insulation, for example, for electric machines is subject to damage and deterioration caused by thermal, electrical, mechanical, chemical and environmental stresses. Typical insulation failure occurs in the slot section between turns or between the coil and ground, and at end windings between coils of adjacent phases. Winding insulation degradation can result in acceleration of machine failure, which decreases the service life of the machine and results in increased costs due to repair or replacement cost and loss of revenue due to machine outage. Therefore it is desirable to monitor the insulation condition for scheduling repair or replacement of winding insulation to prevent such a failure, or at least to anticipate when maintenance or service may become in order.
Off line methods for evaluating the insulation condition include over-voltage hi-pot or high voltage ramp tests, insulation resistance or polarization index tests, surge tests, dissipation factor tests, also known as “tan delta” or power factor tests, and off-line partial discharge tests. Such tests have been extensively used and accepted over many years in providing the condition of the winding insulation. However, these conventional off-line techniques can be intrusive and costly since it is required that the electrical machine be shut down and taken out of service to perform the required diagnostic test and/or measurement. The regular machine maintenance is typically performed once every 3–6 years; therefore, the off-line stator insulation condition cannot be evaluated frequently enough to guarantee reliable operation of the machine until the next outage.
Several on-line measurement techniques are also available for monitoring the winding insulation condition. These include vibration measurement and monitoring, temperature measurement, and differential current measurement. One major drawback of these methods is that the monitoring system detects severe fault conditions only after the fault has occurred, making proactive maintenance and servicing difficult. The on-line partial discharge detects early symptoms of insulation degradation; however, it requires expensive specialized equipment and accurate interpretation of the measurements relies on the skill of an operator.
Accordingly, there is a need for a low cost, simple, and reliable on-line solution for assessing the insulation condition for a DC machine.